1. Field of the Invention
The present invention relates to a method of forming films over the inner circumference (surface) of a cylindrical member and, more particularly, to a method of forming a film consisting of a hard carbon film for enhancing the abrasion resistance of the inner surface of a cylindrical member (part), such as a bushing, a cylinder in which a piston reciprocates or a bearing, and an intermediate film (layer) formed between the hard carbon film and the inner surface of the cylindrical member which is capable of sticking to both the inner surface of the cylindrical member and the hard carbon film with high adhesion.
2. Description of the Prior Art
A hard carbon film is black and has properties similar to those of diamond. A hard carbon film has advantageous properties including a high mechanical hardness, a small friction coefficient with other materials, a high electrical insulation property, a large thermal conductivity and a high corrosion resistance. Accordingly, there have been proposals for coating various devices, including, medical instruments, magnetic heads, tools and such with a hard carbon film.
A hard carbon film is a hydrogenated amorphous carbon film having properties very similar to those of diamond and hence a hard carbon film is often called a diamondlike carbon film (DLC film) or an i-carbon film.
Proposed in JP-A No. 56-6920 is a film forming method of forming a hard carbon film with high adhesion on a surface of a base member. This previously proposed film forming method forms an intermediate film of silicon or a silicon compound over a surface of a base member by sputtering using a gas containing argon gas (Ar gas) and carbon, and then a hard carbon film is formed on the intermediate film.
Such a prior art method of forming a film consisting of an intermediate film and a hard carbon film over the inner surface of a cylindrical member (base member) of carbon tool steel, such as a bushing, will be described with reference to FIG. 10.
Referring to FIG. 10 showing a film forming apparatus for carrying out the prior art method of forming a film consisting of an intermediate film and a hard carbon film overlying the intermediate film, a target 37 of an intermediate film forming material, such as silicon or a silicon compound, and a cylindrical member 11 having a bore 11a defined by an inner surface 11b, are disposed opposite to each other in a vacuum vessel 13.
Gases are removed through a gas outlet port 17 from the vacuum vessel 13 by an evacuating means, not shown, to evacuate the vacuum vessel 13. Then, Ar gas, i.e., a sputtering gas, is supplied through a gas inlet port 15 into the vacuum vessel 13. A negative DC voltage is applied to the target 37 by a first power source 39 and a negative DC voltage is applied to the cylindrical member 11 by a second DC power source 25.
A plasma is thus produced within the vacuum vessel 13 to make the target 37 sputter by bombarding the surface of the target 37 with ions. Consequently, particles of the material forming the target 37 are deposited over the inner surface 11b of the cylindrical member 11 thus forming an intermediate film of silicon or a silicon compound.
Next, a hard carbon film is formed on the intermediate film by a conventional film forming method using an apparatus as shown in FIG. 11.
Referring to FIG. 11, the cylindrical member 11 having its inner surface 11b deposited with the intermediate film is placed i n a vacuum vessel 13 provided with a gas inlet port 15 and a gas outlet port 17. The vacuum vessel 13 is evacuated by an evacuating means, not shown. Then, a gas which contains carbon is supplied through the gas inlet port 15 into the vacuum vessel 13 and the pressure in the vacuum vessel 13 is adjusted to a set pressure.
A positive DC voltage is applied to an anode 31 placed within the vacuum vessel 13 by an anode power source 27, an AC voltage is applied to a filament 33 by a filament power supply 29, and a negative DC voltage is applied to the cylindrical member 11 by a DC power source 25. Thus, a plasma is produced in the vacuum vessel 13 to deposit a hard carbon film on the intermediate film formed over the inner surface of the cylindrical member 11.
The hard carbon film forming method using the apparatus shown in FIG. 11 uses the plasma produced by the DC voltage applied to the cylindrical member 11 and the plasma produced by the filament 33 energized by an AC voltage and the anode 31 energized by the DC voltage. Either the plasma produced around the cylindrical member 11 or the plasma produced around the filament 33 and the anode 31 contributes mainly to hard carbon film formation depending on the pressure in the vacuum vessel 13 during hard carbon film formation.
For example, when the pressure in the vacuum vessel 13 is 3.times.10.sup.-3 torr or above, the plasma produced around the cylindrical member 11 mainly contributes to the decomposition of the gas containing carbon to form the hard carbon film.
Although a carbon film can be formed uniformly over the outer surface of the cylindrical member 11 by the dominant contribution of this plasma, a carbon film formed over the inner surface 11b defining the bore 11a is inferior in adhesion, hardness and quality. This is because the same voltage is applied to the cylindrical member 11, and the inner surface 11b defines a space in which electrodes of the same polarity are disposed opposite to each other, and the plasma prevailing in the bore 11a causes an abnormal discharge called hollow discharge. A hard carbon film formed by hollow discharge is a polymerlike film inferior in adhesion and apt to come off the cylindrical member 11 and have a relatively low hardness.
When the pressure in the vacuum vessel 13 is below 3.times.10.sup.-3 torr, the plasma produced in the neighborhood of the filament 33 and the anode 31 contributes mainly to hard carbon film formation.
Although a hard carbon film can uniformly be formed over the outer surface of the cylindrical member 11 by the dominant contribution of this plasma, the hard carbon film cannot be formed in a uniform thickness with respect to a direction along the axis of the cylindrical member 11 over the inner surface 11b defining the bore 11a. Carbon ions produced by the plasma produced around the filament 33 and the anode 31 are attracted to the surface of the cylindrical member 11 by the negative DC potential of the cylindrical member 11 to deposit the hard carbon film over the surface of the cylindrical member 11.
The hard carbon film is formed by a chemical vapor deposition process when the pressure in the vacuum vessel 13 is above 3.times.10.sup.-3 torr, and the hard carbon film is formed by a physical vapor deposition process when the pressure in the vacuum vessel 13 is below 3.times.10.sup.-3 torr. Therefore, the thickness of the hard carbon film formed over the inner surface 11b of the cylindrical member 11 decreases from the open end of the bore 11a downwards with the depth, which occurs when forming a film by a physical vapor-phase epitaxial growth process, such as a vacuum deposition process, when the plasma produced around the filament 33 and the anode 31 contributes mainly to hard carbon film formation. Consequently, the hard carbon film cannot be formed in a uniform thickness over the entire inner surface 11b of the cylindrical member 11.
Similarly, the thickness of the intermediate film formed over the inner surface 11b of the cylindrical member 11 decreases from the open end of the bore 11a downwards with the depth when the intermediate film is formed by the method previously described with reference to FIG. 10.
FIG. 12 is a graph showing a thickness distribution in an intermediate film formed over the inner surface of a cylindrical member, in which distance from the open end of the cylindrical member is measured on the horizontal axis and thickness is measured on the vertical axis. In FIG. 12, curve a indicates the variation of the thickness of the intermediate film formed by the method described in reference to FIG. 10 with the distance from the open end of the bore of the cylindrical member.
As is obvious from curve a, the thickness of an intermediate film formed by the conventional method decreases sharply from 0.5 .mu.m at the open end of the bore to 0.1 .mu.m at a position 30 mm from the open end.
When a hard carbon film is formed on the intermediate film having such a sharply changing thickness, the portion of the hard carbon film formed in the vicinity of the open end of the cylindrical member sticks to the intermediate film with a high adhesion, whereas the adhesion of the portion of the hard carbon film formed in the depth of the bore decreases with the distance from the open end. Therefore, the portion formed in the depth of the bore is apt to come off.
Since the portion of the intermediate film in the depth of the cylindrical member is thin and is unable to withstand stress induced in the hard carbon film, the intermediate film and the hard carbon film come off the cylindrical member. This is a problem.
Thus, the hard carbon film formed by the conventional method over the inner surface of a cylindrical member cannot fully exercise its advantageous characteristics including high abrasion resistance and high corrosion resistance.